Unvulcanized rubber and/or compounding ingredients therefor are often packaged in thermoplastic film, particularly where the film is in the form of a bag thereof.
A purpose for the film packaging is to minimize loss of material, such as by spillage or by airborne losses, e.g. compounding ingredients for rubber; to minimize a tendency for some of such ingredients to cake together in high humidity situations; and to maximize ease of handling, all of the rubber mixing operation for a rubber products manufacturing facility.
Thus, such packaging of rubber compounding ingredients with a thermoplastic film is often found to be useful for adding compounding ingredients to rubber on a uniform basis.
However, various criteria are important for selecting a suitable thermoplastic film material, particularly a bag thereof, for such purpose.
In one aspect, the film should readily disperse in the rubber to which it is added and mixed.
It is important to appreciate that, for many rubber mixing operations, rubber is mixed with its compounding ingredients in two or more sequential mixing stages. Conventionally, such mixing stages consist of a first, or at least one initial non-productive, mixing stage and a second, or at least one subsequent productive, mixing stage. Additional mixing stages can also be used. In such circumstance, rubber is first mixed in a non-productive mixing stage with its compounding ingredients exclusive of curatives such as sulfur and cure accelerators. In the non-productive mix stage, rubber, carbon black, rubber processing oil and other additives, normally exclusive of curatives. In the non-productive mixing stage, the mixing temperatures are usually relatively high, such as, for example, about 130.degree. C. to about 170.degree. C.
In a subsequent productive mixing stage, sulfur is added to the non-productive mix batch. Processing oil is not conventionally added to the productive mix stage except perhaps a very small amount to reduce the dusting of the ingredients. A lower mixing temperature is used such as, for example, about 90.degree. C. to about 115.degree. C., because of the curatives being present and the mixing can occur over a considerably shorter period of time as compared to the non-productive mixing stage(s).
While various thermoplastic films may be useful for adding basic compounding ingredients to the non-productive mixing stage and which have been found to satisfactorily disperse in the rubber mixture under such circumstances, special considerations have to be given to the productive mixing stage, which is typically although not universally, the second mixing stage.
As heretofore pointed out, the productive mix stage is conventionally operated at a lower temperature (e.g. 110.degree.-120.degree. C.) than that of its preceding non-productive mix stage.
Therefore, for the aforesaid thermoplastic film to be satisfactory for introducing rubber compounding ingredients into the productive mix stage, not only should the film have a softening below the stage's mixing temperature, but it needs to adequately disperse within the compounded rubber at the lower mix temperature and in a relatively short period of time.
Normally, in the non-productive mix stage, about 10 to about 40 phr of processing oil is included.
However, in a more specific case, where a relative high loading of oil is used in the non-productive mix stage for the rubber composition, the productive mix stage is typically operated at a somewhat lower temperature (e.g. 90.degree.-100.degree. C.) and, perhaps more importantly, a lower mix shear is inherently developed due to the highly oil loaded rubber composition.
Thus, with the reduced shear, it becomes more difficult to adequately disperse the aforesaid film in the rubber composition in the productive mix stage over a relatively short period of time.
Thus, the film should quickly melt and flow under the rubber mixing conditions in the productive mix stage which include its mixing temperature and mixing shear forces. Further, the film should disperse into the rubber under the mixing conditions over a very short period of time.
Accordingly, such thermoplastic film should have a softening point (or melting point) below about 70.degree. C. and flow under the aforesaid mixing shear forces in the productive mix stage, including circumstances where a relatively large amount of oil has been mixed with the rubber in the non-productive mix stage which reduces the mixing shear forces.
The softening points of the films has been one of the important factors for their selection as well as other factors, including their relative strength and their compatibility with the rubber and associated resulting compounded rubber properties.
Overall strength of the film and resistance of the film to chemical action of certain compounding ingredients, such ass for example, antioxidants, antiozonants, oleic acids and napthenic oil are also important considerations.
A suitable overall strength of the film can be determined by filling a bag composed of the polycaprolactone film having a thickness of about 2 mils, or 0.05 mm and at about 23.degree. C. with one or more of rubber compounding ingredients without rubber processing oil.
A suitable chemical resistance to certain of rubber compounding ingredients can be determined by filling a bag composed of the polycaprolactone film having a thickness of about 2 mils (0.05 mm) with one or more of oleic acid, non-reactive phenolformaldehyde resin, hexamethoxymelamine and insoluble sulfur/oil blends for rubber compounding purposes storing the filled bag at about 23.degree. C. for about 72 hours and then lifting the bag without supporting its bottom. If the bag does not leak through its film, or the film does not rupture, the film is considered as being acceptable under the chemical resistance test and also the aforesaid strength test. The chemical resistivity of the film is deemed to be not suitable if the film dissolves or leaks it contents through the film itself.
Therefore, for a film to be considered to be suitable for the purposes of this invention, it is required to have a melting point below 70.degree. C., of a suitable overall strength, and a suitable chemical resistivity and an ability to adequately disperse in rubber when mixed therein over a relatively short period of time at a temperature of about 90.degree. to about 105.degree. C.
The aforesaid dispersion requirement extends to highly oil loaded rubber compositions blends to which relative large amounts of oil is added during the mixing of packaged compounding ingredients with the rubber in the non-productive mix stage (e.g. by adding from 60 to 90 phr of oil). The term "phr" relates to parts by weight per 100 parts by weight rubber.
Thus, the selection of a suitable film can only be determined by experimentation concerning its strength, chemical resistivity and dispersion-in-rubber ability under prescribed circumstances. In addition, the film, as a rubber ingredient bag, should not have deleterious effect on the compounded rubber's physical properties such as, for example, tensile strength and elongation.
Polyethylene, while it has been used for rubber packaging and sometimes rubber compounding ingredient packaging in some circumstances, is often considered to be disadvantageous because, when mixed with unvulcanized rubber, it does not usually disperse sufficiently well in the rubber blend at a temperature of about 90.degree. C. to about 105.degree. .
Films of ethylene/vinyl acetate copolymer and of syndiotactic polybutadiene have been used for packaging rubber compounding ingredients for rubber mixing purposes.
Syndiotactic polybutadiene has also been used for such purpose but has not been observed to have a good chemical resistance against oleic acid.
It is still desired to provide films for such packaging purposes which will melt at or below the rubber mixing temperatures and flow under the mixing shear conditions to adequately disperse in the rubber during relatively short mixing times, as well as to exhibit good strength and chemical resistance. The films are, therefore, desired to be suitable for preparing packaged rubber compounding ingredients for mixing with rubber and to provide rubber compounded with such a package.